Causal risk models of air transport - NLR-ATSI

insurance market is strongly influenced by ‘spillover’ from the general insurance market,
the financial markets and overall economy [Swiss Re 1996]. This may have a much bigger
impact than the (expected) crash risk and would thus limit the usebility of a causal risk
model.
Airlines monitor safety performance of the fleet by routinely analysing in-flight recorded
data and checking parameter values for crossings of pre-defined threshold values.
Individual threshold crossings are analysed to determine exactly what happened (see also
section 8.5.4). In addition to this flight data program, most airlines also have a system in
place for the flight crew to report abnormal situations. This information is usually in a freetext
format. The combination of flight data and crew reports provide the airline with a good
picture of individual incidents, but as of yet a tool that allows analysis on a more generic
level does not exist. At best airlines do some sort of trend analysis. A causal risk model
would be very valuable to airlines if it could support providing an overall safety picture
and, most importantly, show airline management the influence of their decisions on the
level of safety. It is therefore required that a causal risk model links-up with the primary
safety data information systems: flight data and safety reports.
A challenge for airlines is to maintain current (safety) standards in a changing world. When
such changes are local and threaten to influence the ‘level playing field’ (see section 4.3.6)
or their competitiveness, airline management will be keen on taking proper
countermeasures to ensure profitability of the company. When locally imposed safety
measures threaten the level playing field, the reaction will be to resist them until and unless
all others are required to implement them also. Competitiveness is the driving factor. A
causal risk model could play a role in analysing future changes, estimating their effect on
safety and providing information that allows selection of proper countermeasures. For
airlines therefore the ability to perform cost benefit analysis of proposed future changes
would be a desirable feature for a causal risk model. This implies also the need for a model
in which the way that future changes in how functions are operationalised can be
incorporated. Because of the low accident frequency, a causal risk model for airlines should
be based on safety indicators other than the accident rate.
4.3.2. Repair stations
Maintenance technicians consider aircraft safety to be the result of their professional skills.
They feel responsible for safety. In the process of releasing an aircraft back into service,
some maintenance technicians even think that they sign off for the safety of the aircraft
rather than (as required by regulation) for having followed prescribed procedures [Biemans
et al 1998]. These procedures are specified in the operator’s maintenance program, which is
normally based on instructions for continued airworthiness prepared by the manufacturer.
The operator may rewrite the structure and format of these maintenance recommendations
to better suit his operation. Once aircraft enter service, the initial maintenance program is
subject to continuous development and update as modifications, product improvements and
operational feedback are incorporated. To evaluate the effectiveness of the maintenance
program and to update it, operators develop a reliability program. The actions resulting
from the reliability program may be to escalate, de-escalate, omit or add maintenance tasks.
By proving to the authority for instance that increasing a servicing or inspection interval for
a particular component does not adversely affect safety, the operator could save money in
maintenance expenditures. The Alaska Airlines MD-83 accident in 2000 (see infobox) is an
example of a case where inspection intervals were extended without the operator (Alaska
Airlines) or the authority (FAA) being sufficiently aware of the safety implications.
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